Recently, introduction of touch panel systems to various kinds of electronic devices has been growing rapidly. For example, the touch panel systems are introduced to portable information devices such as smartphones and automatic vending machines such as automatic ticket machines.
The touch panel system is typically configured to include (i) a display device and (ii) a touch panel stacked on an upper side (front surface) of the display device. Therefore, a sensor provided on the touch panel is likely to be affected not only by a noise such as a clock generated in the display device but also by other noises coming from the outside. Such the noises lead to impairment in detection sensitivity for a touch operation.
Patent Literature 1 describes a touch panel system (coordinates input device) including a countermeasure against such noises. The touch panel system of Patent Literature 1 includes a noise processing section for removing a noise. FIG. 19 is a block diagram illustrating a noise processing section 100 included in the touch panel system of Patent Literature 1. As shown in FIG. 19, the noise processing section 100 includes a filter section 101, a logical inversion section 102, and an adding section 103. The filter section 101 receives an output signal (analog signal) from a sensor provided in a touch panel (not illustrated). The filter section 101 extracts, as a noise signal, an AC signal component included in the input signal. The logical inversion section 102 inverts by 180° the phase of the noise signal thus extracted. The adding section 103 adds, to the input signal which is supplied to the filter section 101 and which includes the noise signal, the noise signal whose phase has been inverted by 180°.
Thus, according to the touch panel system of Patent Literature 1, the noise signal extracted by the filter section 101 is inverted, and the signal thus inverted is added to the input signal (analog signal) supplied from the sensor. Namely, to the noise component included in the input signal supplied from the sensor, such a signal is added which has the same level as the noise component and whose phase has been inverted. This cancels the noise superimposed on the input signal supplied from the sensor. This makes it possible to reduce effects given by the noise included in the input signal supplied from the sensor.
Meanwhile, currently, introduction of a touch panel system into various kinds of electronic device including portable information devices such as smartphones and automatic vending machines such as automatic ticket vending machines is rapidly progressing.
In such electronic devices, there is a demand for a technique for preventing an incorrect operation caused by an unintended contact of an object with a touch panel.
For example, in a case where a user operates such an electronic device while carrying it in a user's hand, contact of the hand holding the electronic device with a touch panel is judged as a touch on the touch panel. Based on this judgment, an incorrect operation would occur in the electronic device. The incorrect operation caused by such a mechanism is expected to occur in a case where the electronic device is a portable apparatus such as a smartphone, a tablet-type terminal, notebook computer, or the like. This reduces convenience of a user.
FIG. 36 is a diagram explaining this mechanism. As illustrated in FIG. 36, when a hand (object) 73 holding a tablet-type terminal 71 makes contact with a touch panel 72 of the tablet-type terminal 71, the contact is judged as a touch on the touch panel 72. This reduces convenience of a user as mentioned above.
The problem of an incorrect operation caused by such a mechanism is expected to more frequently occur as a width of a frame with respect to a display screen in which a touch panel is provided becomes narrower.
Patent Literature 2 discloses a technique for preventing an incorrect operation of continuously detecting a specific coordinate position although no touch is made to a touch panel.
According to an input device disclosed in Patent Literature 2, in a case where detection of a specific coordinate position continues for a predetermined period of time, it is determined whether or not a difference between a detection value based on a change of an electrostatic field between electrodes and a reference value falls in a predetermined update detection range. In a case where the difference falls in the predetermined update detection range, the reference value is updated to the detection value. Note that the reference value is a value which serves as a standard by which presence or absence of a touch is judged based on the detection value. In a case where the detection value in a specific coordinate position is not less than the reference value or in a case where the detection value in a specific coordinate position is above the reference value, it is determined that a touch has been made to the coordinate position.
In this way, according to the input device disclosed in Patent Literature 2, it is possible to prevent an incorrect operation of continuously detecting a specific coordinate position although no touch is made to a touch panel. Moreover, sensitivity of detection may be improved by detecting presence or absence of a touch on the basis of the updated reference value.